API is sort of like the library of methods and classes that the engine uses. The language is the syntax (JS). My problem is the lack of an editor, official documentation (that I can find), or obvious way of adding scripts or editing pre-existing scripts

I see what you mean. I have only written one thing and that's the MMO script 2 years ago. I think you have to just explore and read the code, plus use the console. I find the console extremely powerful and the reason RMMV scripting trumps the others, esp when it comes to objects.